Simple Heat Sensor

The Thermistor, or NTC (Negative Temperature Coefficient) of 10K, is a standard type. Most types will work. The one in the diagram is a 10K model made by Fenwal (#197-103LAG-A01).

The resistance lowers as the surrounding temperature increases which affects the output (pin 6) and energizes the small relay and Led1(optional, just cosmetic and can be left out).

P1 is a regular Bourns trimmer potentiometer and adjusts a certain range of temperatures. I used a 50K, 10-turn type for a bit finer adjustment but any type will work.

Heat Sensor Circuit Diagram:

Heat Sensor

 Table 1:
 
Table of Heat Sensor
 
R2 is optional in case your relays tends to 'chatter' a bit. It provides a bit of hysteresis when the set temperature of the thermistor reaches its threshold point. This value may need to be adjusted anywhere between 120K and 470K (although I indicated different values on the schematic).

Transistor Q1 can be a 2N2222(A), 2N3904, NTE123A, ECG123A, etc. Not critical at all. It acts only as a switch for the relay so almost any type will work, as long as it can provide the current needed to activate the relay's coil.

D1, the 1N4148, acts as a spark arrestor when the contacts of the relay open and eliminates false triggering. Feel free to use any other type, like a 1N4001 or something. Solder directly onto the '+' and '-' relay terminals.

If you need a 'Frost' sensor, just swap positions of the R1 and Th1 positions.


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